The invention described herein relates to the formation of films and materials so produced, and more particularly to the production of films of carbon nanotubes (CNTs).
An ideal nanotube can be thought of as a hexagonal network of carbon atoms, resembling chicken-wire, rolled up to make a seamless cylinder. Typically just a nanometer across, the cylinder can be tens of microns long, with each end “capped” with half a fullerene molecule. Single-wall nanotubes (SWNT) can be thought of as the fundamental cylindrical structure, and these form the building blocks of double-walled nanotubes (DWNT) and multi-walled nanotubes (MWNT), i.e., concentric cylinders of CNT, and the ordered arrays of CNT called ropes.
CNTs and ropes of CNTs are used for making electrical connectors in micro devices such as integrated circuits or in semiconductor chips used in computers because of the electrical conductivity and small size of the carbon nanotube. CNTs are also used as antennas at optical frequencies, and as probes for scanning probe microscopy such as are used in scanning tunneling microscopes (STM) and atomic force microscopes (AFM). In addition, CNTs are useful as electron field-emitters and as electrode materials, particularly in fuel cells and electrochemical applications such as Lithium ion batteries, and CNTs may be used in place of or in conjunction with carbon black in tires for motor vehicles. CNTs are also used as supports for catalysts used in industrial and chemical processes such as hydrogenation, reforming and cracking catalysts, as well as elements of composite materials providing novel mechanical, electrical and thermal conductivity properties to those materials.
Several approaches to the production of CNTs exist in the art. Fullerene tubes, i.e., CNTs, are, for example, produced using carbon arc production methods by which spheroidal fullerenes, i.e., Buckyballs, are produced from vaporized carbon. However, this method only produces multi-walled carbon nanotubes. In other methods, CNTs are produced in a DC arc discharge apparatus using both vaporized carbon and a transition metal from the anode of the arc discharge apparatus.
Another CNT production method, disclosed by PCT/US/98/04513 entitled “Carbon Fibers Formed From Single-Wall Carbon Nanotubes,” produces SWNTs, nanotube ropes, nanotube fibers, and nanotube devices. This method utilizes a laser beam to vaporize material from a target comprising, consisting essentially of, or consisting of a mixture of carbon and one or more Group VI or Group VIII transition metals. The vapor from the target forms CNTs that are predominantly SWNTs. The method also produces significant amounts of SWNTs that are arranged as ropes, i.e., the SWNTs run parallel to each other.
Still another exemplary method of carbon nanotube production is disclosed by PCT US99/25702 entitled “Gas-phase process for production of single-wall carbon nanotubes from high pressure CO.” The method comprises the process of supplying high pressure (e.g., 30 atmospheres) CO that has been preheated (e.g., to about 1000° C.) and a catalyst precursor gas (e.g., Fe(CO)5) in CO that is kept below the catalyst precursor decomposition temperature to a mixing zone. In this mixing zone, the catalyst precursor is rapidly heated to a temperature that results in (1) precursor decomposition, (2) formation of active catalyst metal atom clusters of the appropriate size, and (3) favorable growth of SWNTs on the catalyst clusters. Preferably a catalyst cluster nucleation agency is employed to enable rapid reaction of the catalyst precursor gas to form many small, active catalyst particles instead of a few large, inactive ones. Such nucleation agencies can include auxiliary metal precursors that cluster more rapidly than the primary catalyst, or through provision of additional energy inputs, e.g., from a pulsed or CW laser, directed precisely at the region where cluster formation is desired. Under these conditions SWNTs nucleate and grow according to the Boudouard reaction. The SWNTs thus formed may be recovered directly or passed through a growth and annealing zone maintained at an elevated temperature, e.g., 1000° C., in which tubes may continue to grow and coalesce into ropes.
However, neither PCT/US98/04513 or PCT/US99/25702 provide simple processes for direct formation of thin films of CNTs.
There have been several basic approaches described in the prior art to adapt the CNT production methods discussed above so as to fabricate thin films from CNTs. Examples from the art are in-situ production methods and bulk production methods. In-situ production methods require growth of CNTs on a specially prepared substrate using a carbon vapor; while bulk production methods produce films by starting from a quantity of CNT powder that is then dispersed uniformly onto the target substrate.
In-situ production methods of CNT film have several disadvantages. First, the prepared substrate must be able to withstand high production temperatures, thereby limiting materials that may be used as the substrate. Second, the reactor used to grow the vapor on the prepared substrate must be large enough to house the substrate, which could be impractical for large device applications. Third, assuring the vapor is deposited on the prepared substrate uniformly is difficult, especially over large, complexly shaped or thermodynamically variable areas on the substrate. Finally, scale-up in production is costly for in situ production methods.
The prior art methods for the bulk production of CNT films have several additional problems. Namely, CNTs in bulk produced CNT films are randomly oriented. And, once the CNTs used to produce the films contact one another, they tend to combine to form clumps of tubes that stick together. Those clumps are difficult or impossible to separate from each other prior to dispersal for coating target substrates. Furthermore, some of the CNTs are lost due to inefficiencies in the methods of dispersal and adhesion to the target substrates. And, characteristics of products produced using bulk production methods are not suitable to forming thin films. For example, free-standing foils of CNT, known as “buckypaper,” may be formed by filtration on polymer or other suitable filter material from a CNT sample dispersed in liquid. The “buckypaper” tends to exhibit brittle characteristics, and are therefore useful only for the production of thick CNT films. Furthermore, “buckypaper” formed, as described above, requires a secondary manufacturing step that uses a solvent or carrier solution such as alcohol. The secondary step adds labor costs and any solvent must be completely removed from the product. Also, self-supporting mats of CNT formed by deposition on non-porous substrates placed near the reaction chamber place numerous constraints on the design of the reactor and of the collection system, and even so, result in films that tend to show non-uniform physical properties, corresponding to non-uniform deposition of suspended CNT.
Filters also have been used as substrates for forming thin films, and general literature on filtering by use of belt filters is extensive, and includes both endless type belt filters (such as in U.S. Pat. No. 4,057,437) and supply rolls of filter medium which feed clean filter material through the system on a one-time basis, so that the filter medium moves from the supply spool to a take-up spool once the filter is loaded to its target maximum level (such as in U.S. Pat. No. 4,011,067). However, such prior art filtering systems do not provide a means to remove the accumulated filtered material leaving its uniform dispersion characteristics intact. Since orientation of the filtered material after its removal from the filter generally is not of concern to designers of filtration systems, prior art filters do not accommodate thin or thick film transfer procedures which preserve the as-deposited filtered material physical configuration.
In one prior art method that does seek to form thin films of CNTs on filters, the films are made by filtering a light aerosol, i.e., 0.005 g/ml, of nanotubes (such as those produced by the HiPco method of CVD production) suspended in air. Unfortunately, the filtration does not occur from the reactor exhaust directly, and there is no suggestion in the art to make the filtration a continuous process.
References to the use of adhesive substrates in the formation of CNT films are also limited in the prior art. In work described in R. Baughman et al, Science, 284, 1340 (1999), the authors disclose a method of constructing an actuator by attaching a layer of “buckypaper” to a piece of tacky adhesive tape. However, the buckypaper employed by Baughman was a thick film produced by post-processing of as-produced CNT. Second, no effort is made by Baughman to provide a means to remove the tacky adhesive tape, i.e., to produce a pure CNT structure. Another prior art reference, Chang et al. (U.S. Pat. No. 6,436,221), describes the application of adhesive tape to thin nanotube films formed from a CNT slurry and subsequent detachment of the tape to remove poorly attached CNTs. However, the purpose of the method taught by Chang et al. is to improve the quality (smoothness) of films left bonded to the original substrate, not to recover the nanotubes that adhere to the adhesive.
Thus, a need exists for a method that forms thin films of CNTs on arbitrary substrates more efficiently.